Download THE TRINIDADIAN GUPPY Guppies (Poecilia reticulata) are small

THE TRINIDADIAN GUPPY
Fig. 1 Female guppies hiding among plants.
Guppies (Poecilia reticulata) are small,
freshwater fish, native to the island nation of
Trinidad and Tobago and to the adjacent
South American mainland. Although guppies
have been introduced to tropical regions and
pet stores worldwide, they exist in a ‘natural
laboratory’ within the streams of Trinidad.
Guppies have become a model system in
Behavior, Ecology, and Evolution due to their
wealth of easily observable behaviors and
their ability to rapidly evolve in response to
changing environmental pressures.
In guppies, the females are larger and
drab in color, while the males are smaller and
brightly colored. Guppies are unusual among
fish because they are livebearers. This
means that they give birth to live young,
rather than laying eggs like most fish. The
time from birth to sexual maturity in guppies
is about 3 months, and females can give birth
to 3-30 babies. In fish, a group of sibling
babies is referred to as a brood.
Many of the differences in behavior and
morphology seen in distinct populations of
guppies are associated with differences in
predation pressure. In downstream “highpredation” environments guppies co-occur
with many predators. These predators are
prevented from moving upstream by waterfall
barriers,
resulting
in
“low-predation”
populations at higher elevations. Highpredation guppies have repeatedly colonized
and adapted to upstream low-predation
environments resulting in parallel changes in
life-history traits, morphology, and behavior
(see Table 1 at the end of this packet). Thus,
this evolutionary scenario has been naturally
replicated many times and is unique because
it allows us to better understand why certain
characteristics are favorable in different
environments and how these characteristics
evolve. In essence, we have access to a
natural experiment that has been replicated
many times.
To understand how and why differences
among populations evolve, we have to think
in terms of natural selection (also called
survival of the fittest), an idea first proposed
by Charles Darwin (1809-1882). The central
idea behind this theory is that animals in the
wild must overcome the challenges to survive
and reproduce in changing environments.
Only those individuals that best match their
environment survive and pass on their genes
to the next generation. As this process
repeats itself, advantageous characteristics
become more common and undesirable
characteristics become less common. Over
time this persistence and spread of
advantageous traits allows populations to
evolve and become adapted (well-matched)
to their environments. The theory of natural
selection is a central principle in evolution
and biology in general.
Fig. 2 A male (top right) and female (bottom
left) guppy.
Pet traders have taken advantage of
guppies’ ability to rapidly evolve by selecting
for very elaborate colors and tail shapes.
Selective breeding or artificial selection is
similar to natural selection, except that it is
the breeders who decide which traits are
desirable,
rather
than
the
natural
Guppy Behavior
Fig. 3 Typical high-predation (top) and lowpredation (bottom) sites.
environment. Selective breeding has factored
heavily into many aspects of human society
(domestic animals, agricultural products,
etc.). The fancy guppies we find in pet
stores originated from wild guppies, but many
years of selective breeding have changed
both their appearance and their behavior.
Studies have demonstrated that fancy
guppies have lost many of the innate fear
responses seen in wild guppies, making them
beautiful but unlikely to survive in the wild.
Fig. 4 The pike cichlid, Crenicichla frenata, a
common and voracious guppy predator.
Anti-predator behavior
Shoaling behavior is a grouping
behavior used as an anti-predator tactic by
many fish species. It is less coordinated than
schooling behavior, but serves the same
purpose of creating safety in numbers. Highpredation guppies shoal more than lowpredation guppies, and guppies from both
populations increase shoaling behavior if they
detect a predator in their immediate
environment. Nearly all fish display a special
escape response known as a ‘fast-start’
escape response. Guppies can perform this
behavior so quickly that it is only visible when
recording with a high-speed camera. When
watching with the naked eye we see the
guppy move from one position to another
without any apparent movement in between.
Other predator evasion behaviors displayed
by guppies include freezing and/or hiding to
make themselves less visible to predators.
Courtship behavior
The mating display of the male guppy is
the ‘sigmoid display’, in which a male will
position himself perpendicularly in front of a
female, assume the S-shaped posture that
gives the display its name, and quiver his
body. This display is basically a dance to
show off and get the female’s attention. The
problem is that sigmoid displays also make
males more visible to predators. Similarly,
females tend to prefer brighter males, but
bright coloration also makes the males more
conspicuous to predators. In addition, studies
have found that females prefer rare males,
males that are daring (i.e. that they have
observed in the presence of a predator), and
males that are preferred by other females (i.e.
that they have observed mating with other
females). Together, these factors create a
trade-off between getting mates and avoiding
predators, something that makes sexual
selection important in guppies.
Sexual selection is a special case of
natural selection that is focused on
differences between individuals in their ability
to acquire mates and produce offspring.
Sexual selection is an important extension of
natural selection because it helps explain the
presence of traits that seem maladaptive –
such as long tail feathers in birds and bright
colors in guppies. In other words, traits that
are bad for predator avoidance can still
become common if they are good for mating
success. In fact, some of these traits are also
thought to be indicators of good genes. For
example, bright colors in guppies are an
indicator of good health (males become dull
when they are sick), and a bright male guppy
that survives to reproduce despite being more
visible may be particularly good at predator
avoidance.
Foraging behavior
As with most animals, finding food takes
up much of guppies’ lives. As with mating
behavior, there is a trade-off between
foraging and avoiding predators. Guppies
looking for food might be distracted and not
as aware of predators. Being in a group can
help, since there are more eyes to look for
predators, but being in a group also increases
competition for food and mates. Because
guppies have to balance these different
factors, they will alter their behaviors
depending on how hungry they are. Hungrier
guppies take more risks than guppies that
have recently eaten.
Fig. 5 A male guppy tries to get the attention of
a female.
Fig. 6 Our favorite guppy predator. He is a pike
cichlid named Pickles who lives in our lab.
High-predation
Low-predation
body size
smaller
bigger
body shape
streamlined
robust
# of babies
many
few
size of babies
smaller
larger
time to maturity
short
long
# sigmoids
few
many
# sneaky attempts
many
few
amount of color
less
more
shoaling
often and close
not often, less close
time foraging
less
more